Hexagonal boron nitride (h-BN) takes a stable phase having planar graphite structure of SP2 bonds. Hexagonal boron nitride single crystal has superior electrical insulation, a low dielectric constant, heat resistance, chemical stability and lubricating property. It has thus been expected as a material for use in connection with electronics and machinery. In particular, boron nitride has a large band gap of about 6 eV and is thus excellent as a material for a substrate for luminous devices (LED, LD) in the ultraviolet wavelength region. The development of a production technique for a single crystal wafer has been demanded.
It is, however, very difficult to grow bulk single crystal hexagonal boron nitride. This is because hexagonal boron nitride hardly vaporizes until a high temperature and does not easily dissolve, and a flux appropriate for growing hexagonal boron nitride has not been found.
A production technique for growing hexagonal boron nitride single crystal by flux process (solution process) was disclosed in Japanese Patent publication No. 2001-72499A.
It has been further known that aluminum nitride single crystal is obtained by heating aluminum nitride and calcium nitride in sealed state at, for example, 1550° C. or 1610° C. (Mat. Res. Bull. Vol. 9 (1974) pages 331 to 336).
According to Mat. Res. Bull. Vol. 9 (1974) pages 331 to 336, an aluminum nitride material and a calcium nitride material are mixed and then heated to successfully precipitate single crystal aluminum nitride. Such calcium containing flux is, however, extremely high in reactivity, so that it is susceptible to reacting with the crucible material to melt the crucible. It is further described that the sealed crucible may be broken as the inner pressure is increased. Due to these problems, the growth of boron nitride single crystal using a calcium-series flux has not been studied until the present invention was made.
A method of growing hexagonal boron nitride single crystal of a high quality (low defect density) and of a large size has not been known yet. Thus, a novel growing method has been demanded.